It has heretofore been difficult to couple signals in discrete optical paths across the interface between relatively rotatable members without suffering relatively high signal losses. Such losses are usually so high that active electronic circuits are required to restore suitable signal levels after coupling through the joint. In a U.S. Pat. No. 4,027,945 to M. L. Iverson, a so-called optical slip ring includes plural bundles of optical fibers coaxially arranged in concentric cylindrical shapes for coupling across an interface between relatively rotatable bodies.
A. H. Fitch shows in U.S. Pat. No. 4,165,913 a multichannel coupler in which signals injected into each of plural optical fibers wrapped around a rotatable shaft at spaced locations are unidirectionally coupled through roughened outer axial surfaces of the fibers to respective stationary, signal detecting heads.
The U.S. Pat. No. 4,401,360 to G. L. Streckmann et al. teaches an optical slip ring arrangement in which multiple, unidirectional electro-optical signal translations are used to couple respective lightguide paths across an interface between relatively rotatable members. Each path so coupled extends between a position off the axis of rotation on one side of the interface and a position on the axis of rotation on the other side of the interface.